Powdered iron core



Patented Aug. 7, 1951 POWDERED IRON CORE Harold T. Faus, Lynn, Mass, assignor to General Electric Company, a corporation of New York No Drawing. Application July 20, 1949, Serial No. 105,905

3 Claims.

This invention relates to magnetic materials and cores, and more particularly to powdered iron magnetic cores of an improved type suitable for use in high frequency circuits. The invention is concerned more particularly with a process for producing such cores, which process provides a solution to the problem of effectively insulating the particles of iron one from another.

Prior teachings in the art suggest a considerable number of processes for the manufacture of magnetic cores and employ many different components. Cores produced in accordance with these teachings are generally intended to be used at relatively low radio frequencies, for instance, in the range from 400 to 1600 kilocycles, and have resistivities in the order of several hundred ohms. My invention is concerned with cores intended for use at frequencies in the range from to 50 megacycles and upwards. The resistivity of core material at such frequencies must be in the order of several megohms per centimeter cube. I

Accordingly, it is an object of my invention to provide a powdered iron core having a higher resistivity than heretofore possible, and which is suitable for use at frequencies in the range from 25 to 50 megacycles or higher.

Another object of my invention is to provide a method for producing rust-proofed powdered iron cores and for effecting an increased measure of insulation of the constituent iron particles, one from the other, than heretofore possible.

Further objects of my invention will become apparent to those skilled in the art as a reading of this specification proceeds.

The principles of my invention may best be understood from a description of my method of preparing cores from powdered carbonyl iron powder. While this refers to one specific product and method, it will serve to indicate the general process which may be followed with suitable variations in the component substances.

Carbonyl iron is iron produced by the decomposition of iron carbonyl, a compound of iron and carbon monoxide. Under certain conditions of temperature and pressure, the compound is converted into iron and carbon monoxide. Since this change occurs when the iron carbonyl is gaseous, the iron is precipitated in very small spherical particles which have excellent magnetic properties. Iron particles thus produced are termed carbonyl iron, and since this substance is well known, it need not be described further.

In accordance with my invention, 1020 grams of carbonyl iron powder are mixed in 300 cubic centimeters of water to which a wetting agent has been added. By a wetting agent, is meant a substance which reduces the surface tension of water, such as, for instance a dioctyl ester of sodium salt of sulfosuccinic acid. The mixture is stirred until the iron powder is thoroughly wetted, and 60 grams of lead acetate are then added and the mixture stirred until the lead acetate is dissolved.

The mixture is then stirred into a solution consisting of 120 grams of sodium dichromate dissolved in 900 grams of water. The lead acetate and sodium dichromate thereupon react to produce lead chromate which is precipitated and deposited on the iron particles, while sodium acetate remains in the solution. That part of the lead chromate which does not adhere to the iron particles remains in suspension in the water. The mixture is then washed in two changes of water, to the extent of approximately one liter per washing. The mixture is allowed to settle after each washing and the water is poured off. The washing is then repeated with two changes of alcohol in the same manner.

The wet mixture consisting of carbonyl iron powder with lead dichromate deposited thereon, is then mixed with a phenolic condensation product or resin, for example a liquid phenol-formaldehyde resin in the amount of 135 grams. The mixture of lead chromate coated carbonyl iron powder and phenol resin is then dried by continuous heating and stirring under a blast of heated air. A partial polymerization of the phenol resin occurs and the mixture becomes thicker. When it has reached the consistency of thick putty, the mixture is crumbled into coarse particles and heated at to centigrade until it is solid enough to be powdered after subsequent cooling to room temperature.

The powder resulting from the crushing of the coarse particles is then placed in suitable molds and compressed at a pressure of approximately 50,000 pounds per square inch into cores of suitable size and shape, depending upon the subsequent utilization of the material. No heat is applied to the cores during the compression. This is an important feature of my method, since it has been found that the simultaneous application of heat and pressure while forming the powder into cores tends to decrease the resistivity of the material. This appears to be caused by a rupturing or break-down of the lead chromate coating on the iron particles through the simultaneous application of heat and pressure as practiced in the prior art. In accordance with my method a partial polymerization of the phenol resin is initially obtained at a temperature of from 90 to 100 as previously mentioned. The formation of the powder into a core then occurs without heat and since the resin is already partially polymerized, solidification into the desired core shape takes place with practically no breakdown of the insulation between iron particles.

After the cores have been formed, they are removed from the pressure mold and baked at a moderately low temperature, for instance 100 centigrade for a sufiicient time to complete the polymerization. Since there is no pressure applied to the core during this second polymerization step, the lead chromate insulation remains efiective and the protective coating on the iron particles is not ruptured.

Powdered iron cores produced in accordance with my invention were found to have a resistivity of 4 megohms per centimeter cube. This represents a tremendousadvance over the resistivity figure of 50 to 500 ohms per centimeter cube obtained with powdered iron cores produced in accordance with methods previously described, wherein it is claimed that each individual iron particle is coated with a thin film of solid but not brittle insulating substance. The use of lead chromate as I have described permits the coating on the iron particles to act as both an electrical insulator and a rust inhibitor. Actual tests made by exposing cores, produced in accordance with my invention, to the weather, demonstrated that these cores do not rust even under adverse weather conditions.

While a specific process and material has been described, it will, of course, be understood that various modifications may be made without departing from the invention. The appended claims are therefore intended to cover any such modifications within the true spirit and scope of the invention.

' What I claim as new and desire to secure by Letters Patent of the United States is:

1. A magnetic substance consisting of fine spherical particles of carbonyl iron, an insulating andv rust-resisting coating of lead chromate on said particles, and a binder of a polymerized phenol formaldehyde resinous condensation product holding said particles in a solid mass.

2. The method of making magnetic cores composed of carbonyl iron particles, which consists of mixing said particles in water to which a wetting agent has been added, stirring sodium dichromate into said mixture, adding thereto a lead acetate solution to cause'lead chromate to form and precipitate on said particles, washing said particles, mixing said particles with a liquid phenol formaldehyde resinous condensation product, heating to cause a partial polymerization of the resinous product, crumbling and powdering said partially polymerized product, forming the resulting powder under pressure to a desired shape, and finally heating the desired shape to complete the polymerization of the resinous product.

3. The method of making magnetic cores composed of carbonyl iron particles, which consists of mixing said particles in water to which a wetting agent has been added, stirring sodium dichromate into said mixture in sufiicient quantity to form a fluid paste, adding to said paste a solution of lead acetate to cause lead chromateto form and precipitate on said particles, washing REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Andrew May 15, 1928 Number 

1. A MAGNETIC SUBSTANCE CONSISTING OF FINE SPHERICAL PARTICLES OF CARBONYL IRON, AN INSULATING AND RUST-RESISTING COATING OF LEAD CHROMATE ON SAID PARTICLES, AND A BINDER OF A POLYMERIZED PHENOL FORMALDEHYDE RESINOUS CONDENSATION PRODUCT HOLDING SAID PARTICLES IN A SOLID MASS. 